st: Problem with TOSTRING command

Dear list members,
I am trying to reformat variables that are in numbers format to string
format, therefore I'm using TOSTRING command to do that. Most of the
variables change when I run the do files to string format, but some of them
do not. I realize that the problem could be because the numbers in the
variables are too big therefore here is the sentence that I created
tostring Units104, replace format(%20.0f)
Now when we run in that time, the sentences was working fine, but now I was
try to run in others files and did not work.
Somebody know other alternative to do this?
Thank you,
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/